1. Field of the Invention
The present invention relates to a reference signal measurement method and an apparatus for use in a wireless communication system including plural base stations with distributed antennas. More particularly, the present invention relates to a reference signal measurement method of a terminal for efficient downlink transmission in a mobile communication system including plural base stations, each having a plurality of antennas distributed in the service area thereof based on a Distributed Antenna System (DAS).
2. Description of the Related Art
Mobile communication systems have evolved into high-speed, high-quality wireless packet data communication systems that provide data and multimedia services beyond the early voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) defined in the 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in the 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined by the Institute of Electrical and Electronics Engineers (IEEE), have been developed to support the high-speed, high-quality wireless packet data communication services. More particularly, LTE is a communication standard developed to support a high speed packet data transmission and to maximize the throughput of the radio communication system with various radio access technologies. LTE-A is the evolved version of LTE to improve the data transmission capability.
LTE is characterized by 3GPP Release 8 or 9 capable base station and terminal (i.e., a user equipment) while LTE-A is characterized by 3GPP Release 10 capable base station and user equipment. As a key standardization organization, 3GPP continues standardization of the next release for more improved performance beyond LTE-A.
The 3rd and 4th generation wireless packet data communication systems of the related art (such as HSDPA, HSUPA, HRPD, and LTE/LTE-A) adopt Adaptive Modulation and Coding (AMC) and Channel-Sensitive Scheduling techniques to improve the transmission efficiency. AMC allows the transmitter to adjust the data amount to be transmitted according to the channel condition. For example, the transmitter is capable of decreasing the data transmission amount for bad channel condition so as to fix the received signal error probability at a certain level or increasing the data transmission amount for good channel condition so as to transmit large amount of information efficiently while maintaining the received signal error probability at an intended level. Meanwhile, the channel sensitive scheduling allows the transmitter to serve the user having a good channel condition selectively among a plurality of users so as to increase the system capacity as compared to allocating a channel fixedly to serve a single user. This increase in system capacity is referred to as a multi-user diversity gain. In brief, the AMC method and the channel-sensitive scheduling method are methods for receiving partial channel state information being fed back from a receiver, and for applying an appropriate modulation and coding technique at the most efficient time determined depending on the received partial channel state information.
In a case of using AMC along with a Multiple Input Multiple Output (MIMO) transmission scheme, it may be necessary to take a number of spatial layers and ranks for transmitting signals in to consideration. In this case, the transmitter determines the optimal data rate in consideration of the number of layers for use in a MIMO transmission.
Recently, many researches are being conducted to replace Code Division Multiple Access (CDMA) used in the legacy 2nd and 3rd mobile communication systems with Orthogonal Frequency Division Multiple Access (OFDMA) for the next generation mobile communication system. The 3GPP and 3GPP2 are in the middle of the standardization of an OFDMA-based evolved system. OFDMA is expected to provide superior system throughput as compared to CDMA. One of the main factors that allow OFDMA to increase system throughput is the frequency domain scheduling capability. As channel sensitive scheduling increases the system capacity using the time-varying channel characteristic, Orthogonal Frequency Division Multiplexing (OFDM) can be used to obtain more capacity gain using the frequency-varying channel characteristic.
FIG. 1 is a graph illustrating a relationship between time and frequency resources in an LTE/LTE-A system according to the related art.
Referring to FIG. 1, the radio resource for transmission from the evolved Node B (eNB) to a User Equipment (UE) is divided into Resource Blocks (RBs) in the frequency domain and subframes in the time domain. In the LTE/LTE-A system, an RB consists of 12 consecutive carriers and occupies 180 kHz bandwidth.
Meanwhile, a subframe consists of 14 OFDM symbols and spans 1 msec. The LTE/LTE-A system allocates resources for scheduling in unit subframe in the time domain and in unit of RB in the frequency domain.
FIG. 2 is a time-frequency grid illustrating a single resource block of a downlink subframe as a smallest scheduling unit in an LTE/LTE-A system according to the related art.
Referring to FIG. 2, the radio resource is of one subframe in the time domain and one RB in the frequency domain. The radio resource consists of 12 subcarriers in the frequency domain and 14 OFDM symbols in the time domain, i.e., 168 unique frequency-time positions. In LTE/LTE-A, each frequency-time position is referred to as a Resource Element (RE).
The radio resource structured as shown in FIG. 2 can be used for transmitting plural different types of signals as follows:
Cell-specific Reference Signal (CRS): a reference signal transmitted to all the UEs within a cell,
Demodulation Reference Signal (DMRS): a reference signal transmitted to a specific UE,
Physical Downlink Shared Channel (PDSCH): a data channel transmitted in downlink which the eNB uses to transmit data to the UE and mapped to REs not used for reference signal transmission in data region of FIG. 2,
Channel Status Information Reference Signal (CSI-RS): a reference signal transmitted to the UEs within a cell and used for channel state measurement. Multiple CSI-RSs can be transmitted within a cell, and
Other control channels (such as a Physical Hybrid Automatic Repeat reQuest (ARQ) Indicator Channel (PHICH), a Physical Control Format Indicator Channel (PCFICH), a Physical Downlink Control Channel (PDCCH)): channels for providing control channel necessary for the UE to receive PDCCH and transmitting ACKnowledgement/Non-ACK (ACK/NACK) of Hybrid ARQ (HARQ) operation for uplink data transmission.
In addition to the above signals, zero power CSI-RS can be configured in order for the UEs within the corresponding cells to receive the CSI-RSs transmitted by different eNBs in the LTE-A system. The zero power CSI-RS (muting) can be mapped to the positions designated for CSI-RS, and the UE receives the traffic signal skipping the corresponding radio resource. In the LTE-A system, the zero power CSI-RS is referred to as muting. The zero power CSI-RS (muting) by nature is mapped to the CSI-RS position without transmission power allocation.
In FIG. 2, the CSI-RS can be transmitted at some of the positions marked by A, B, C, D, E, F, G, H, I, and J according to the number of antennas transmitting CSI-RS. In addition, the zero power CSI-RS (muting) can be mapped to some of the positions A, B, C, D, E, F, G, H, I, and J. The CSI-RS can be mapped to 2, 4, or 8 REs according to the number of the antenna ports for transmission.
For two antenna ports, half of a specific pattern is used for CSI-RS transmission, for four antenna ports, an entirety of the specific pattern is used for CSI-RS transmission, and for eight antenna ports, two patterns are used for CSI-RS transmission.
Meanwhile, muting is always performed by pattern. For example, although the muting may be applied to plural patterns, if the muting positions mismatch CSI-RS positions, it cannot be applied to one pattern partially.
In the case of transmitting CSI-RSs of two antenna ports, the CSI-RSs are mapped to two consecutive REs in the time domain and distinguished from each other using orthogonal codes. In the case of transmitting CSI-RSs of four antenna ports, the CSI-RSs are mapped in the same way of mapping the two more CSI-RSs to two more consecutive REs. This is applied to the case of transmitting CSI-RSs of eight antenna ports.
In a cellular system, the reference signal has to be transmitted for downlink channel state measurement. In the case of the 3GPP LTE-A system, the UE measures the channel state with the eNB using the CSI-RS transmitted by the eNB.
The channel state is measured in consideration of a few factors including downlink interference. The downlink interference includes the interference caused by the antennas of neighbor eNBs and thermal noise that are important in determining the downlink channel condition. For example, in the case that the eNB with one transmit antenna transmits the reference signal to the UE with one receive antenna, the UE has to determine the amount of energy per symbol that can be received in downlink and the amount of interference that may be received for the duration of receiving the corresponding symbol to calculate Es/Io from the received reference signal. The calculated Es/Io is reported to the eNB such that the eNB determines the downlink data rate for the UE.
In the typical mobile communication system, the base station apparatus is positioned at the center of each cell and communicates with the UE using one or more antennas deployed at a restricted position. Such a mobile communication system implemented with the antennas deployed at the same position within the cell is referred to as a Centralized Antenna System (CAS). In contrast, the mobile communication system implemented with plural Remote Radio Heads (RRHs) belonging to a cell are distributed within the cell area is referred to as the DAS.
FIG. 3 is a diagram illustrating an antenna arrangement in a distributed antenna system according to the related art.
Referring to FIG. 3, there are distributed antenna system-based cells 300 and 310. The cell 300 includes five transmission points including one high power transmission position 320 and four low power transmission positions 340, 350, 360, and 370. In this case, each transmission point may be provided with one or more transmit antennas.
The high power transmission point is capable of providing at least minimum service within the coverage area of the cell while the distributed low power transmission points are capable of providing UEs with the high data rate service within a restricted area. The low and high power transmission points are all connected to a central controller 330 so as to operate according to the scheduling and radio resource allocation of the central controller 330.
In the distributed antenna system, one or more antennas can be deployed at one geometrically separated antenna position. In the present invention, the antenna(s) deployed at the same position is referred to as an RRH. In the distributed antenna system depicted in FIG. 3, the UE receives signals from one geometrically distributed transmission point and regards the signals from others as interference.
In the distributed antenna system of FIG. 3, all antennas of one cell participate in the CRS transmission. For example, all the antennas of the cell transmit the CRS simultaneously or mute at the REs on which the CRSs of other antennas are mapped to avoid interference. In the case that all the antennas of one cell transmit the CRS, the data signal transmission based on the CRS is performed through all the antennas of the cell.
Meanwhile, in a case which some antennas of the cell participate in the CRS transmission, the other antennas not participated in the CRS transmission mute transmission at the REs on which the CRS is mapped to avoiding interference to the CRS transmitted through other antennas. In this, the data signal transmission based on the CRS is also performed through antennas participating in CRS transmission.
In the case of CSI-RS, the antennas of one cell may transmit different CSI-RSs. For example, although the same CRS is transmitted through the transmission points 320, 340, 350, 360, and 370, the different CSI-RSs may be transmitted through the transmission points 320, 340, 350, 360, and 370. In order for the transmission points to transmit different CSI-RSs, it can be considered to map the CSI-RSs to different CSI-RS transmission resource positions in the grid of FIG. 2 or applying different scrambling codes to the CSI-RS transmission resource positions.
In the LTE-A system, the UE feeds back the information on the downlink channel state for use in downlink scheduling of the eNB. For example, the UE measures the reference signal transmitted by the eNB in downlink and feeds back the information estimated from the reference signal to the eNB in the format defined in LTE/LTE-A standard. In LTE/LTE-A, the UE feedback information includes the following three indicators:                Rank Indicator (RI): number of spatial layers that can be supported by the current channel experienced at the UE,        Precoding Matrix Indicator (PMI): precoding matrix recommended by the current channel experienced at the UE, and        Channel Quality Indicator (CQI): maximum possible data rate at which the UE can receive a signal in the current channel state. CQI may be replaced with the Signal to Interference plus Noise Ratio (SINR), maximum error correction code rate and modulation scheme, or per-frequency data efficiency that can be used in a similar way to the maximum data rate.        
The RI, PMI, and CQI are associated among each other in meaning. For example, the precoding matrix supported in LTE/LTE-A is configured differently per rank. Accordingly, the PMI value ‘X’ is interpreted differently for the cases of RI set to 1 and RI set to 2. In addition, when determining CQI, the UE assumes that the PMI and RI which it has reported is applied by the eNB. For example, if the UE reports RI_X, PMI_Y, and CQI_Z, this means that the UE is capable of receiving the signal at the data rate corresponding to CQI_Z when the rank RI_X and the precoding matrix PMI_Y are applied. In this way, the UE calculates CQI with which the optimal performance is achieved in real transmission under the assumption of the transmission mode to be selected by the eNB.
In LTE/LTE-A, the RI and PMI disabled mode (hereinafter, RI/PMI disabled mode) in which only CQI is fed back without RI and PMI is supported. The UE operating in RI/PMI disabled mode feeds back only CQI without RI and PMI.
FIG. 4 is a diagram illustrating feedback patterns in an RI/PMI enabled mode and an RI/PMI disabled mode of the UE according to the related art.
Referring to FIG. 4, the UE transmits RI, PMI, and CQI in the RI/PMI enabled mode as follows:                transmit RI at timings 400 and 430        transmit PMI at timings 410 and 440        transmit CQI at timings 420 and 450        
When the RI/PMI is in a disabled state, the UE does not transmit RI and PMI but transmits only CQI at timings 460 and 470. The reason for disabling RI and PMI transmission is to decrease the feedback overhead, resulting in improvement of uplink system throughput and reduction of UE's battery consumption.
In the LTE/LTE-A system, the UE operating in RI/PMI disabled mode, the UE always measures CRS to generate CQI.
As described above, the UE generates CQI under the assumption of a specific transmission mode. In the LTE/LTE-A system, the UE operating in RI/PMI disabled mode generates CQI under the assumption that the eNB applies Space Frequency Block Code (SFBC) precoding when it uses two transmit antenna. In the case that the eNB uses four transmit antennas, the UE generates CQI under the assumption that Frequency Shift Time Diversity (FSTD) precoding is applied.
In the LTE/LTE-A system, the CRS measurement-based CQI generation method of the UE operating in the RI/PMI disabled mode has two problems.
First, the CRS measurement-based method is not appropriated for the distributed antenna system as shown in FIG. 3. Since the CRS is transmitted through plural transmission points within one cell, the CQI acquired based on the CRS measurement is applicable only when the PDSCH is transmitted from all the transmission points of the cell to one UE simultaneously. Typically, PDSCH transmission is performed only through the transmission point closest to the UE in the distributed antenna system. In the case of transmitting the PDSCH through the closest transmission point, other transmission points may transmit PDSCH to other UEs, resulting in improvement of system throughput. In order to generate accurate CQI on downlink channel per transmission point in the distributed antenna system, it is required to measure CSI-RS other than CRS.
Secondly, CRS supports up to 4 transmit antennas. Accordingly, although it is possible to apply aforementioned SFBC or FSTD precoding to 2 or 4 CRS antennas per transmission point, each transmission point of the distributed antenna system may use up to 8 antennas. Since there is no precoding scheme defined for the case that each transmission point has 8 antennas, CRS cannot be used in this case. In addition, since SFBC and FSTD precodings are defined in consideration of only CRS, it is difficult to apply the precodings for the CSI-RS.
In the related art, the above problem can be addressed in such a way that the UE operating in RI/PMI disabled mode generates the CQI based on the CSI-RS measurement.
However, the problem caused by the CRS measurement is not the problem occurring always but when the distributed antenna system is introduced in the LTE/LTE-A mobile communication system of the related art. For example, if the distributed antenna system is not applied, it may be advantageous to generate CQI based on CRS measurement in the RI/PMI disabled mode. However, it has no capability to determine whether the distributed antenna system is configured, the UE cannot measure the CRS and CSI-RS selectively.
Therefore, a need exists for a reference signal measurement method and an apparatus for use in a wireless communication system including plural base stations with distributed antennas.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.